1. Field of the Invention
The present invention relates to cold rolled steel useful for deep drawing purposes and to the alloying of such steel with small quantities of copper, chromium and nickel whereby the adhesion between the steel surface and the zinc layer during hot galvanizing is improved.
2. Description of the Prior Art
A conventional steel suitable for deep drawing is described in SIS 141147 (Swedish Industrial Standard). In order to obtain the best properties for deep drawing, a cold rolled and coiled sheet or strip is annealed in a bell-type furnace or the like at a temperature of 650.degree. to 700.degree. C. for approximately 20 hours. Thereafter the sheet is temper rolled. The deep drawing suitability of a steel is a function of its "R-value" as fully described in published Swedish Patent Application Ser. No. 375,326. The above-described heat treated sheet of SIS 141147 steel generally has an R-value of about 1.5.
A conventional method for hot galvanizing comprises subjecting, in a continuous sequence, a cold rolled and coiled steel strip first to an annealing process in a furnace where the steel becomes soft and easy to handle and then to a galvanizing process. Then the steel strip is coiled again onto a capstan. After such treatment, heat treated sheets of SIS 141147 steel which before treatment had an R-value of about 1.5 are characterized by reduced deep drawing properties and often have an R-value of 1.0 or less.
Another known steel suitable for deep drawing contains small quantities of carbon and manganese, preferably not more than about 0.02 and 0.04 weight percent respectively, and an amount of titanium which generally should not exceed 0.4 weight percent. Generally such titanium-stabilized steels will contain from 0 to about 0.02 weight percent carbon, from 0 to about 0.04 weight percent manganese and from about 0.01 to about 0.4 weight percent titanium. Preferably, however, the titanium content should be in the range of from about 0.15 to about 0.3 weight percent. Unlike the heat treated SIS 141147 steel mentioned above, these latter steels retain their good deep drawing properties after hot galvanizing and it has been found that if the strip is passed in continuous sequence through an annealing furnace at 800.degree. to 900.degree. C., subsequently cooled to 450.degree. to 500.degree. C. and then passed through a hot galvanizing bath, the sheet will retain deep drawing properties corresponding to an R-value of 1.5 or greater.
When the heat treated SIS 141147 steel is hot galvanized, no substantial problems have been encountered in the adhering of the zinc to the surface of the sheet. On the other hand, when the latter described titanium alloy sheet is hot galvanized, it has been found that the rejection rate due to insufficient adhesion of the zinc is high.